The US space agency's $820m Mars exploration rovers Spirit and Opportunity set out for Mars in 2003 and, once landed in 2004, got busy looking for signs of water, a critical building block for life, on the Red Planet.

But while the Jet Propulsion Lab's pair were driving around, they left criss-crossing tracks in the Martian sand, which appear to sketch out a crude drawing of an erect manhood and testicles.

It is thought the image was an accident, rather than the work of a puerile space boffin. A Redditor spotted the cosmic willy last night, prompting a huge surge of traffic to NASA's website - which promptly crashed.

The agency's boffins lost contact with the Spirit rover in 2010 after it got stuck in the sands, but Opportunity is still going strong, wandering around a creator named Endeavour and heading towards an area dubbed Botony Bay.